Various studies, including our own, demonstrate a role for the extracellular calcium-sensing receptor (CaR) in the regulation of renal calcium handling. Population studies have suggested a role for genetic variants in CaR in contributing to the human variation in renal calcium excretion. Here we will further explore the role of CaR in the kidney, with a focus on proximal tubule function. We will use two complementary mouse models as well as human studies to further these goals. In Aim 1, we will focus on the role of CaR in the proximal tubule. We will use our existing CaR deficient model to examine the role of CaR in specific aspects of proximal nephron physiology;elements of this study will be extended to mice generated in Aim 2 with proximal nephron-specific ablation of CaR. Prior data suggest that extracellular calcium modulates 1a- hydroxylation of vitamin D by the proximal tubule, hence we will assess the regulation of 1a-hydroxylase expression and function in CaR-deficient mice. As luminal calcium in the proximal nephron affects the reabsorption of several solutes, we will examine the role of CaR in regulating proximal tubular fluid and solute transport using tubule perfusion, confocal immunofluorescence, and eventually transport assays in primary cell culture. In Aim 2, we will develop a mouse harboring a "floxed" CaR allele. We will use proximal tubule Cre transgenic mice to allow us to target CaR inactivation to this renal segment. These mice will be used in Aim 1 studies also. Whole animal studies in this new model will help clarify the in vivo contributions of proximal tubule CaR to kidney function. In Aim 3, we will study common variation in human CaR with Projects 4 and Core A. We will perform functional studies to assess the biological effects of known and the newly identified non-synonymouscSNPs (including effects on signaling, cell-surface expression, and dimerization). We will genotype DMA and assay CaR transcript expression using a series of human lymphoblastoid cell lines in order to determine the correlation between genetic haplotype and CaR expression level. This last "Subaim" will serve as a pilot for possible further exploration of haplotype/expression correlation studies of other genes in Core A.